United States urged restraint one day after Ukrainian President Petro Poroshenko warned a 'Russian invasion' was possible

US Vice President Joe Biden urged both Russia and Ukraine to show restraint one day after Ukrainian President Petro Poroshenko warned he could not rule out "a full-scale Russian invasion."

The White House said Biden spoke with Poroshenko by phone on August 19 and "relayed that the United States had sent a message to Russia that the world is watching and underscored the need to de-escalate the situation."

Biden "also urged Ukraine to show restraint," the White House said, while both leaders expressed concern over a recent surge in fighting with pro-Russian separatists in eastern Ukraine.